This invention relates to the use phosphoric acid esters, in particular mixtures of mono- and di-esters of orthophosphoric acid and hydroxyl-terminated, oxyethylated long-chain alcohols or oxyethylated alkyl phenols or derivatives thereof, as surface modifiers for water insoluble silver carboxylate nanoparticles. The nanoparticles are used in aqueous oxidation-reduction imaging forming compositions that include the nanoparticles and a reducing agent. The carboxylates are typically silver salts of long chain fatty acids that are used to formulate imaging forming compositions that are useful in aqueous photothermographic or thermographic imaging elements.
Photothermographic materials are well known in the photographic art. Photothermographic materials are also known as heat developable photographic materials. The photothermographic materials, after imagewise exposure, are heated to moderately elevated temperatures to produce a developed image in the absence of separate processing solutions or baths. The heat development can provide a developed silver image in the photothermographic material.
Thermographic materials are similar except that there is no photosensitive material present. Images are formed by direct imagewise heating.
An example of a known photothermographic silver halide material comprises (a) a hydrophilic photosensitive silver halide emulsion containing a peptizer with (b) an organic solvent mixture, (c) a hydrophobic binder and (d) an oxidation-reduction image-forming composition. The oxidation-reduction imaging forming composition typically comprises (i) a silver carboxylate that is usually a silver salt of a long-chain fatty acid, such as silver behenate or silver stearate, in combination with (ii) an organic reducing agent, such as a phenolic reducing agent. It has been desirable to have hydrophilic photosensitive silver halide emulsion containing a peptizer in such a photothermographic material because of the higher photosensitivity of the silver halide emulsion and the ease of control in preparation of the emulsion based on conventional aqueous silver halide emulsion technology.
A problem has been encountered in preparing these photothermographic silver halide materials. This problem involves the mixing of a hydrophilic photosensitive silver halide emulsion containing a peptizer with an oxidation-reduction imaging forming composition. The imaging forming composition contains hydrophobic components including a hydrophobic binder, such as poly(vinyl butyral), and a silver salt of a long-chain fatty acid, such as a silver salt of behenic acid. Typically, when the hydrophilic photosensitive silver halide emulsion is mixed with the hydrophobic imaging forming materials and then coated on a suitable support to produce a photothermographic element, the resulting element produces a less than desired degree of photosensitivity, contrast and maximum density upon exposure and heat processing. This problem has been encountered in photothermographic silver halide materials, as described in, for example, U.S. Pat. No. 3,666,477 of Goffe, issued May 30, 1972. Goffe proposed addition of alkylene oxide polymers and a mercaptotetrazole derivative to the photothermographic material to help provide increased photosensitivity.
In addition, a variety of organic solvents have been proposed in order to help prepare a photothermographic silver halide composition containing the described image-forming components. The organic solvents that have been proposed include isopropanol, acetone, toluene, methanol, 2-methoxyethanol, chlorinated solvents, acetone-toluene mixtures and certain non-aqueous polar organic solvents. The described individual solvents, such as isopropanol, have not provided the desired improved properties. There has been a continuing need to provide improved relative speed and contrast with desired maximum image density.
Agfa EPA 0 848 286 published Jun. 17, 1998, discloses a thermosensitive element comprising silver behenate, an organic reducing agent therefor in thermal working relationship therewith and a binder, characterized in that the silver behenate is not associated with mercury and/or lead ions. While a surfactant is used during the preparation of the silver behenate, large crystals result.
U.S. Pat. No. 3,887,597 issued Jun. 3, 1975 to Ohkubo et al describes the preparation of a silver salt of an organic acid in the presence of a phosphoric ester solvent. However, the phosphoric ester is not water-soluble and serves as a solvent for the organic acid before precipitation. Also, before use and before admixture with other components, the phosphoric ester solvent is removed and the silver salt of an organic acid is isolated. (Col 5 lines 52 through 59) After isolation and washing, the salt is incorporated into an organic coating solution along with other components. See for example the coating composition of Example 4 using isopropyl alcoholxe2x80x94methanolxe2x80x94acetonexe2x80x94methyl cellosolve.
As noted in the discussion of the ""597 patent above, traditional photothermographic elements have been coated from organic solvents. It would be highly desirable to be able to produce an aqueous based element.
In one aspect of the invention, there is provided an oxidation-reduction imaging forming composition comprising an aqueous based dispersion of (i) nanoparticulate silver carboxylate particles having on the surface of the particles a surface modifier which is a phosphoric acid ester and (ii) an organic reducing agent.
In another aspect of the invention, there is provided a photothermographic composition comprising an aqueous based dispersion of a) a photosensitive silver halide emulsion containing a peptizer and b) an oxidation-reduction imaging forming composition comprising (i) nanoparticulate silver carboxylate particles having on the surface of the particles a surface modifier which is a phosphoric acid ester and (ii) an organic reducing agent. The described aqueous photothermographic composition can be coated on a support to provide a useful photothermographic element.